The invention relates to a polyolefinic multilayer film comprising a base layer containing polypropylene, and at least one outer layer.
Sensitive products (foodstuffs, chocolate and cookies) and the increasing speed of modern packaging machines require films having a low minimum heat-sealing temperature in combination with excellent passage through the machine. The prior art discloses films which have minimum heat-sealing temperatures of around 100.degree. C., in some cases even as low as from 87 to 88.degree. C. Numerous publications relate to improvements in the passage of these films through the machine.
EP-A-0 114 312 and EP-A-0 114 311 describe transparent or opaque polypropylene films having a minimum heat-sealing temperature of below 100.degree. C. and good passage through the machine. These films have an outer layer comprising a mixture of C.sub.2 /C.sub.3 /C.sub.4 terpolymer and C.sub.3 /C.sub.4 copolymer as well as a low-molecular-weight resin, polypropylene homopolymer and polydiorganosiloxane. According to this teaching, the wide heat-sealing range of the film and the low minimum heat-sealing temperature are achieved by the olefin resin composition of copolymer and terpolymer. The minimum heat-sealing temperature is 90.degree. C. However, the incorporation of hydrocarbon resins into the outer layer causes resin deposits on the stretching rolls during production. These deposits cause streaking, and thus, the rolls must be cleaned frequently.
EP-A-0 480 282 describes a biaxially oriented multilayer polyolefin film which can be heat-sealed at low temperatures on both sides. Its base layer contains a peroxidically degraded propylene homopolymer, and its outer layers contain mixtures of C.sub.2 /C.sub.3 /C.sub.4 terpolymer and C.sub.2 /C.sub.3 copolymer and also a combination of SiO.sub.2 and polydiorganosiloxane. The polydialkylsiloxane has high viscosity and is added in the form of a masterbatch. The minimum heat-sealing temperature is 88.degree. C.
The prior art also discloses heat-sealable outer layers made of C.sub.2 /C.sub.3 copolymers as well as those made of C.sub.2 /C.sub.3 /C.sub.4 terpolymers. All these films have minimum heat-sealing temperatures of above 100.degree. C.